Gel-free buffer tube with adhesively coupled optical element

ABSTRACT

In a buffer tube arrangement, discrete domains of adhesive material provide adhesive coupling of a water-swellable element and an optical fiber bundle that includes a plurality of optical fibers.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. application Ser. No.11/424,112, filed Jun. 14, 2006 (and published Jan. 25, 2007, as U.S.Patent Application Publication No. 2007/0019915 A1), which itself claimsthe benefit of U.S. Provisional Application No. 60/700,739, filed Jul.20, 2005; U.S. Provisional Application No. 60/710,654, filed Aug. 24,2005; and U.S. Provisional Application No. 60/764,776, filed Feb. 3,2006. This application further claims the benefit of pending U.S.Provisional Application No. 60/886,235, filed Jan. 23, 2007. Theforegoing commonly assigned applications and publication are herebyincorporated by reference in their entirety.

FIELD OF THE INVENTION

The present invention relates to a cable that includes a water-swellabletape or yarn that is used for water-blocking inside a buffer tube andthat further includes an adhesive material for bonding the tape or yarnto the optical fibers and/or the buffer tube.

BACKGROUND

Water-blocking, petroleum-based filling gel in optical fiber buffertubes and cable serves two purposes. First, by completely filling all ofthe free space inside a buffer tube containing an optical fiber or fiberbundle, the filling gel blocks the ingress of water into the cable.Second, being a thixotropic material, the filling gel mechanicallycouples the optical fiber or fiber bundle to the buffer tube andprevents the optical fiber or fiber bundle from pulling back inside thebuffer tube as the buffer tube is processed, as the cable is installedor otherwise handled, or as the cable is subjected to thermally induceddimensional changes due to environmental exposure. A totally dry cableeliminates the filling gel from inside a buffer tube, whether loose tubefiber or ribbon cable. In a totally dry cable, filling gel may bereplaced by a water-swellable element, which may be a tape or a yarncarrying a water-swellable material. The water-swellable element,however, may not provide for sufficient coupling of the optical fiber orribbon to the buffer tube, thus allowing the optical fiber or ribbon topull back inside the tube/cable when the cable is installed or exposedto temperature extremes.

U.S. Pat. No. 6,970,629 and U.S. Patent Application Publication No.2005/0213902 A1 disclose adhesively attaching a water-swellable element(or dry insert) to a buffer tube. Although this may assist in preventingthe optical fibers or ribbons from retracting with respect to the buffertube, it introduces its own problem in the manufacturing of the cable.More specifically, it is preferable that the water-swellable elementhave the ability to move with respect to the buffer tube prior to andduring cooling of the buffer tube, so that the optical fibers or ribbonsdo not experience a column-like bending when the buffer tube contractsduring the cooling. Such bending of the optical fibers or ribbons mayresult in optical signal losses because the optical fibers or ribbonsmay touch the inside wall of the buffer tube.

Indeed, it is common to tension the optical fibers or ribbons to inducea positive strain during the buffer-tube-extrusion process. In thismanner, when the buffer tube material contracts on cooling, the positivestrain in the optical fibers or ribbons compensates for some of thebuffer tube contraction before the optical fibers or ribbons begin toabsorb the contraction by the column-like bending.

SUMMARY OF THE INVENTION

In one aspect, the invention embraces a buffer tube arrangement thatincludes a plurality of optical fibers; a buffer tube enclosing theoptical fibers; a water-swellable element (e.g., tape or yarn) disposedbetween the optical fibers and the buffer tube; and a curable (e.g., achemically cross-linking) adhesive material provided on thewater-swellable element for bonding the water-swellable element to thebuffer tube. The curable adhesive material may be, for example, athermally curable material, such as a two-component silicone or aradiation-curable material (e.g., a silicone acrylate cross-linked byexposure to actinic radiation).

The optical fibers may be, for example, formed into ribbons havingbetween two and 24 or more optical fibers held together in a planarconfiguration by the application of a thin overcoat, or matrix, of UVcurable material. Thus, in the buffer tube, optical fibers may bebundled as a ribbon stack, or they may be individually helically woundaround each other. Alternatively, the optical fibers may be looselydisposed.

In another aspect, the buffer tube arrangement can include an additionaladhesive material that bonds or otherwise couples the optical fibers tothe water-swellable material (e.g., a water-swellable tape or yarn).

The foregoing, as well as other objectives and advantages of theinvention and the manner in which the same are accomplished, is furtherspecified within the following detailed description and itsaccompanying, non-limiting drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of an embodiment of a first aspect of thecables;

FIG. 2 is a sectional view of another embodiment of a first aspect ofthe cables;

FIG. 3 is a sectional view of an embodiment of a second aspect of thecables;

FIG. 4 is a sectional view of an embodiment of a third aspect of thecables;

FIG. 5 is a sectional view of an embodiment of a fourth aspect of thecables; and

FIG. 6 is a sectional view of an embodiment of a fifth aspect of thecables.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 illustrates an embodiment of a first aspect of the cables,wherein a buffer tube arrangement 10 includes a plurality of opticalfibers 12 arranged in a bundle, a water-swellable element 14 disposedaround the bundle of optical fibers 12, a buffer tube 16 in which theoptical fibers 12 and water-swellable element 14 are enclosed, a curableadhesive material 18 for bonding the water-swellable element to thebuffer tube, and an adhesive material 20 for bonding the optical fiberbundle 12 to the water-swellable element 14.

As previously noted, although FIG. 1 illustrates the optical fibersbundled together as a ribbon stack, the optical fibers, alternatively,may be loosely disposed or helically wound around each other, forexample.

The water-swellable element 14 may include a water-swellable tape orwater-swellable yarn that carries or contains a water-swellable powder.The water-swellable powder may be applied to both sides of thewater-swellable element, or it may be applied to only one side. Incertain embodiments, water-swellable element 14 may include two layers,and the water-swellable powder may be applied between the two layerssandwiched together.

The composition of the water-swellable tape or yarn is not particularlylimited. An example tape or yarn may be made of a material whichincludes a polyester, such as, for example, an inexpensive, strong andlightweight polyester carrier material within which particles orcoatings of sodium polyacrylate (i.e., an exemplary superabsorbentpolymer or SAP) are affixed. Exemplary tapes may also be spun-bondedfilaments of polyester, although other materials are sometimes used(e.g., polypropylene). Other examples of water-swellable tapes that canbe used in the cables according to the present invention are disclosedin U.S. Pat. Nos. 4,909,592, 6,284,367, 6,899,776, and 7,231,119.Examples of water-swellable yarns, including a plurality (e.g., four) of1500-denier water-swellable yarns, that can be used in the cablesaccording to the present invention are disclosed in U.S. Pat. Nos.4,703,998, 6,633,709, and 6,654,526. Each of the foregoing patents ishereby incorporated by reference in its entirety.

The composition of buffer tube 16 is also not particularly limited andmay include, for example, polypropylene, polyethylene, or polybutyleneterephthalate.

The compositions of buffer tube 16 and water-swellable element 14 may besuch that the two do not sufficiently bond together through normalbonding mechanisms, such as, for example, melt-bonding (diffusionmechanism), Van der Waal's forces or hydrogen bonding (electrostaticmechanisms), surface energy interactions, and other electrostaticmechanisms. This may be the case when, for example, a polyester-basedwater-swellable element 14 is used together with a polypropylene buffertube 16.

Depending on the materials chosen for the compositions of buffer tube 16and water-swellable element 14, a slight amount of mechanical couplingbetween buffer tube 16 and water-swellable element 14 may occur due tofibrils of the water-swellable element 14 becoming embedded in the innerwall of the buffer tube 16. This may occur, for example, during coolingof the buffer tube 16. This kind of mechanical coupling may be minimizedor maximized by design, and if minimized, may be insufficient to ensureadequate coupling forces between the optical fibers 12 and the buffertube 16. For example, if, during installation the cable is overlyjostled or otherwise roughed up, a polyester-based water-swellableelement, which merely has its fibers embedded in the inner wall of apolypropylene buffer tube 16, may separate from the buffer tube 16.

Thus, using a water-swellable tape or water-swellable yarn towater-block inside a buffer tube 16 may significantly reduce couplingforces between the optical fiber bundle 12 (e.g., optical fiber ribbons)and the buffer tube 16, by relying mainly on friction between theoptical fiber bundle 12, the tape or yarn (i.e., the water-swellableelement 14), and the buffer tube 16.

Therefore, a curable adhesive material 18 may be provided on the surfaceof or embedded in the water-swellable tape or water-swellable yarn tobond, upon curing, the water-swellable element 14 to the buffer tube 16.The curable adhesive material 18, for example, may be athermally-curable material or a radiation-curable material and can beapplied in an unfoamed state as shown in FIG. 1 or a foamed state asshown in FIG. 2. Although an adhesive material 20 for bonding theoptical fiber bundle 12 to the water-swellable element 14 is notillustrated in FIG. 2, embodiments of the first aspect of the cableswherein curable adhesive material 18 is in a foamed state may alsoinclude such an adhesive material 20.

Foaming may reduce stress on components of the cable, including theoptical fibers 12, by allowing the buffer-tube-extrusion process to pushthe foamed curable adhesive thoroughly around the water-swellableelement 14 without deformation of the buffer tube 16. Also, the linearstresses which can build in the optical fibers 12 (e.g., the ribbon) dueto temperature changes and differential thermal contraction between thecurable adhesive material and the ribbons, are reduced because of thefoam structure. A foamed curable adhesive material may be closed-cell innature.

Application of curable adhesive material 18 in a foamed state mayeffectively block water between water-swellable element 14 and thebuffer tube wall, while the water-swellable element 14 may effectivelyblock the interstice between itself and the ribbons.

An example of a thermally curing material which may be used as curableadhesive material 18 is a two-part, room temperature vulcanizing (RTV)silicone, such as GE RTV615, which may be foamed before application inthe buffer tube. Another example is a RTV silicone that is self-foaminga few seconds after application, such as, for example, Dow Corning's3-8159 RF silicone foam. Another example is a two-part urethane, whichalso may be foamed during mixing or which can be formulated to self-foamduring the curing process. Examples of the latter are Reoflex urethanerubbers sold by Smooth-On Corp. These kinds of materials can also beformulated with a very low modulus that does not change substantiallywith temperature excursions normally encountered in the environments inwhich optical fiber cables may be deployed.

Examples of radiation-curable materials include Shin-Etsu's OF212 andOF207 radiation-curable silicone acrylates, and DSM Desotech's3471-1-135 radiation-curable urethane acrylate. Such materials arefast-curing with applied actinic radiation and typically possess lowYoung's modulus to minimize stress on the optical fibers or fiberbundles.

The method for applying the curable adhesive material 18 is notparticularly limited. For example, curable adhesive material 18 may beapplied as a continuous bead or an intermittent bead. Also, a pluralityof beads may be applied (e.g. two beads, intermittent or continuous, acertain number of degrees apart, such as 180°).

The curable adhesive material 18 may be applied, for example, to theoutside of the wrapped water-swellable element 14 (i.e., containing theoptical fibers 12 or ribbon stack on the inside) just before enteringthe extruder for the buffer tube. The curable adhesive material 18 maystill be liquid for the rest of the buffer-tube-application process,allowing the water swellable element 14 and the optical fibers 12 toremain together in sync (i.e., moving at the same rate) all the way tothe take-up reel.

After the buffer-tube-application, the curable adhesive material 18 maybe completely cured, forming a bond between the buffer tube 16 and thewater-swellable element 14 wrapped around the optical fibers 12. Forexample, curing of adhesive material 18 may be designed to take placeafter the buffer tube 16 has cooled to a predetermined degree or at apredetermined time after extruding the buffer tube 16.

The application arrangement of curable adhesive 18 may be adjusted toprovide the degree of bonding required. In this regard, the adhesivebond provided by curable adhesive 18 preferably should be such that itis possible to cut the buffer tube 16 near an end and pull it off overthe water-swellable element 14 with relative ease. On the other hand,the bonding provided by curable adhesive 18 preferably should providefor coupling of the water-swellable element 14 to the buffer tube 16 forsuccessful handling/installation of the cable without the optical fibers12 or ribbons in the buffer tube 16 retracting within the cable.

An advantage of using a curable adhesive material is that a tension canbe applied to both the water-swellable material 14 and the opticalfibers 12 during the buffering process, providing the water-swellablematerial 14 the ability to move with respect to the buffer tube 16 priorto the curing of the adhesive material.

As noted, the ability to subject the optical fibers and water-swellablematerial to tension is beneficial in certain manufacturing methods. Morespecifically, it may be necessary to limit the excess length of theoptical fibers inside the buffer tube in order to minimize opticalsignal losses caused by contact stresses between the optical fibers andthe inside wall of the buffer tube. The excess length is generated whenthe thermoplastic buffer tube contracts linearly during post-extrusioncooling because the optical fibers do not contract. To limit the excesslength, it is common to tension the optical fibers at payoff and thefiber-containing buffer tube after the coupling point to induce apositive strain during the buffer-tube-extrusion process. Then, when thebuffer tube material contracts on cooling, the positive strain in theoptical fibers compensates for some of the buffer tube contractionbefore the optical fibers begin to absorb the contraction by controlledbuckling. The water-swellable element 14 can also be subjected to astrain to prevent wrinkling or bunching of the water-swellable element14 when the buffer tube contracts on cooling.

The buffer tube arrangements described herein may allow for theelimination of the filling gel and yet still provide a desirablecoupling profile between the water-swellable element and the buffertube. The buffer tube arrangements can also employ cured adhesion,rather than friction, to guarantee coupling even in short sections ofcable. The efficacy of the friction approach is questionable. Theachievement of sufficient frictional force between the contents of thebuffer tube and the buffer tube wall may be detrimental to the opticalperformance of the optical fibers.

An adhesive material 20 may be used to bond the optical fiber bundle 12to the water-swellable element 14. For example, the adhesive material 20may be applied during the buffer tube application process as a singlebead or a plurality of beads from a fixed head positioned directly overthe optical fibers between the die controlling the positioning of theoptical fibers and the water-swellable element applicator. In otherembodiments, adhesive material 20 may be embedded in water-swellableelement 14 in a manner that will permit it to bond the water-swellableelement 14 to the optical fibers.

The die here may be an apparatus that positions the optical fibers(e.g., arranges all the ribbons together into a stack). The die andribbon payoffs may be in a “tree” that rotates while everything else isfixed. Thus, the ribbon stack may be twisted helically as it is fed intothe buffer tube extruder head. Accordingly, a bead of adhesive material20, in effect, may spiral around the ribbon stack, passing over thecorners and over each side of the stack before the water-swellable tapeis applied. Alternatively, the bead applicator may be mounted on arotating fixture such that the bead or beads may be applied only ontothe corners of the ribbon stack, or onto the top and bottom of theribbon stack, or only onto the sides of the ribbon stack.

The particular application arrangement of adhesive material 20 is notintended to be limited to a single bead. For example, two beadspositioned a certain number of degrees apart, such as, for example, 180°apart, may be applied to help keep the stack better centered andwell-cushioned against contact with the water-swellable element andbuffer tube wall and to maintain stack integrity. The adhesive material20 may also be applied as a film directly onto the water-swellable tapebefore it is wrapped around the ribbon stack. Other shapes and numbersof adhesive beads may be utilized as well. In every case, the intentionis to couple the ribbon stack to the water-swellable tape through theuse of the adhesive.

Advantageously, the adhesive may be foamed (e.g., silicone foam) tofurther reduce the modulus of the material. The degree of foaming,measured by percent reduction in adhesive density from the fullynon-foamed state, may be, for example, from one percent to 95 percent.More preferably, the degree of foaming may be 20 percent to 80 percent.Most preferably, the degree of foaming may be from 40 percent to 60percent. The foamed adhesive material may fill from one percent tovirtually 100 percent of the free space inside the water-swellableelement, more preferably from one percent to 50 percent of the freespace, most preferably from 1 percent to 20 percent of the free space.

In another embodiment, the foam may be applied as small discrete domainsof the adhesive material 20, such as silicone foam, sputtered ontowater-swellable element 14. In yet another embodiment, the foam may beapplied as small, discrete domains of the adhesive material 20 sputteredonto the optical fiber bundle 12 (e.g. a ribbon stack of opticalfibers). Under either embodiment, the minutely localized application ofthe adhesive foam may be generated, for example, by the addition of gasfrom an external source, by a chemical reaction internal to thematerial, or by the addition of material that expands due to heat. Byway of specific example, the adhesive material 20 may be dispersed ontothe optical elements using a high velocity gas.

Such techniques, which will be understood by those having ordinary skillin the art, can achieve small dots (i.e., discrete domains) of adhesivefoam that can provide satisfactory adhesive coupling of thewater-swellable element 14 and the optical fiber bundle 12. Forinstance, dots of adhesive material (e.g., having diameters averagingless than one millimeter or so) may be, on a relative basis, widelydispersed (e.g., spaced more than about one millimeter from adjacentdots of adhesive material). Those having ordinary skill in the art willunderstand that the application of adhesive material 20 should besufficient to develop the desired adhesive coupling between thewater-swellable element 14 and the optical fiber bundle 12 (i.e., lessadhesive material and wider dispersion yields a relatively weaker bond).According to this aspect of the invention, adhesive bonding typicallyprovides the predominant coupling force between the water-swellableelement 14 and the optical fiber bundle 12.

As with other application methods, the particular application of theadhesive material 20 is not intended to be limited to a single method.The foamed adhesive material 20 may be applied (e.g., sprayed) to theoptical fiber bundle 12 and/or the water-swellable element 14 in one ormore controlled streams. This can regularly distribute the adhesivematerial 20 over all or part of the optical fiber bundle 12 and/or thewater-swellable element 14 to achieve, for instance, a pattern ofdiscontinuously spaced adhesive material 20. Alternatively, the foamedadhesive material 20 may be applied less systematically to the opticalfiber bundle 12 and/or the water-swellable element 14 thereby yielding amore random, discontinuous application of adhesive material 20.

The step of applying adhesive material may be performed at various timesduring the fabrication process. The adhesive material may be applied(e.g., sputtered) onto the water-swellable tape or the ribbon stackswell before the water-swellable tape and the ribbon stack are coupled.More typically, the adhesive material is applied to the water-swellabletape (or to the ribbon stack) as the water-swellable tape is formedaround the ribbon stack. Preferably, the discontinuous adhesive materialis integrated to the water-swellable element 14 so that, during theapplication of the buffer layer (e.g. extrusion of the buffer tube 16),shearing forces between the optical fiber bundle 12 and thewater-swellable element 14 will not remove the adhesive material 20. Ofcourse, any application method is acceptable provided the adhesivefunctions to couple the optical fiber bundle 12 (e.g., the ribbon stack)and the water-swellable element 14 (e.g., the water-swellable tape).

One advantage of employing discrete domains of the adhesive material(e.g., discontinuous foam dots) is that it provides adequate coupling ofthe optical fiber bundle 12 and the water-swellable element 14 yetminimizes physical contact with the optical elements. In other words, itis thought that reducing the surface area of the adhesive material thatcontacts the optical elements will reduce the potential formicrobending.

The composition of adhesive material is not particularly limited. It mayinclude a hot melt adhesive, such as, for example, an ethylene-vinylacetate, an ethylene-butyl acetate, or an ethylene-ethyl acetate, or itmay include styrene-butadiene block copolymer blends with oils,thermoplastic urethane, a thermoset urethane or other thermosetcomposition, or a polyvinyl butyral. Adhesive material 20 may eveninclude a polypropylene or another polyolefin.

For example, adhesive material 20 may include a block copolymer ofstyrene end-blocks with a butylene or an ethylene-butylene mid-block.The styrene end-blocks on the molecules may associate as the rubber inthe blend cools, forming pseudo-crosslinks and giving the blend thecharacteristics of a cross-linked solid while remaining soft through abroad temperature range. An example of such an adhesive is MA-145 soldby Master Adhesive.

Adhesive material 20 may also be a curing material, such as, forexample, any of the radiation-curable materials or thermally curingmaterials discussed herein with respect to curable adhesive material 18.

According to the first aspect of the cables, a curable adhesive material18 and adhesive material 20 are used to bond the water-swellable element14 to the buffer tube 16 and the optical fiber bundle 12 to thewater-swellable element 14, respectively. However, the cables are notlimited to this particular aspect.

For example, according to a second aspect of the cables, an embodimentof which is shown in FIG. 3, cables may be provided in which thewater-swellable element 14 is adhered to the buffer tube 16, either byfriction or by mechanically coupling the buffer tube 16 directly to thewater-swellable element 14.

In this second aspect, the compositions of buffer tube 16 andwater-swellable element 14 may be chosen to promote the coupling of thewater-swellable element 14 to a wall of buffer tube 16 through themechanical interlocking of fibers embedding into the tube wall. Fibrilsof the water-swellable element 14 may become embedded in the inner wallof the buffer tube 16, for example, during cooling of the extrudedbuffer tube. In an embodiment of this second aspect of the cables, thewater-swellable element 14 may be made of a material including, forexample, polyester fibers and the buffer tube 16 may be made of amaterial including polyethylene.

According to the second aspect of the cables, the cables include anadhesive material 20 for bonding the optical fibers to thewater-swellable material. Adhesive material 20 may be any of theadhesive materials 20 previously identified in connection with FIG. 1.

On the other hand, the cables do not require the presence of theadhesive material 20 for bonding the optical fibers to thewater-swellable material.

In this regard, according to an embodiment of a third aspect of thecables, illustrated in FIG. 4, there is provided a buffer tubearrangement 10 in which a curable adhesive material 18 is provided forbonding a water-swellable element 14 to a buffer tube 16, but adhesivematerial 20 for bonding optical fibers 12 to the water-swellablematerial 14 is not included.

This third aspect of the cables relies on frictional coupling of theoptical fibers (e.g., the fiber bundle) to the water-swellable elementin order to couple the optical fibers to the buffer tube wall.

FIG. 5 illustrates an embodiment of yet another aspect (i.e., a fourthaspect) of the cables. According to the fourth aspect, the buffer tubearrangement 10 includes a plurality of optical fibers 12 arranged in abundle (e.g., a ribbon stack), a water-swellable element 14 disposedaround the bundle of optical fibers 12, and a buffer tube 16 in whichthe optical fibers 12 and water-swellable element 14 are enclosed.

As in the first aspect illustrated in FIG. 1, an adhesive material 20may be used for bonding the bundle of optical fibers to thewater-swellable element 14. Although FIG. 5 illustrates the opticalfibers bundled together as a ribbon stack, the optical fibers may beloosely disposed or helically wound around each other.

The fourth aspect differs from previous aspects in that a fibrousmaterial 22 is provided between the water-swellable element 14 and thebuffer tube 16. For example, a web of fibrous material 22 may behelically wrapped around the water-swellable element containing theoptical fibers. An advantage of this aspect is that the water-swellableelement may comprise a material of like composition with the buffer tubematerial. The water-swellable element may include, for example,polypropylene spun-bonded webs capturing water-swellable powder betweentwo layers. A polyester spun-bonded web interposed between such awater-swellable element and a polypropylene buffer tube prevents thewater-swellable element from developing an undesirably high level ofbonding with the buffer tube. Again, this arrangement may allow thefiber bundle and water-swellable element to be placed under strainduring the buffer-tube-extrusion process; the web of fibrous materialallows slip between the water-swellable element and the buffer tubeduring this dynamic process. There may be at least two advantages toutilizing a water-swellable element comprising two layers of webcapturing water-swellable powder between. First, the water-swellableparticles may be kept away from the optical fibers, reducing themicrobending induced by the particles impinging on the optical fibers.Second, little or no loose powder may be encountered when a cablecontaining such a two-layer water-swellable tape is accessed.

The buffer tube arrangement according to the fourth aspect includes acurable adhesive material 24 for bonding the fibrous material 22 to thewater-swellable element 14. Curable adhesive material 24 may be any ofthe adhesive materials identified before in connection with curableadhesive material 18 of FIGS. 1, 2, and 4. Curable adhesive 24 may beapplied, for example, on the surface of fibrous material 22 betweenwater-swellable element 14 and fibrous material 22 prior to entering thebuffer tube extruder, on the surface of water-swellable element 14between water-swellable element 14 and fibrous material 22 prior toentering the buffer tube extruder, or curable adhesive material 24 maybe embedded into water-swellable element 14 and/or fibrous material 22prior to entering the buffer tube extruder. Curable adhesive material 24may be applied as a bead or in any of the manners identified before inconnection with curable adhesive material 18 of FIGS. 1, 2, and 4.

Fibrous material 22 may be, for example, a fibrous synthetic resin, suchas a spun-bonded web or fabric. Fibrous material 22 may includespin-and-calender fibers that have been randomly oriented, pressed flat,and bonded with heat, thereby remaining porous and of a fibrous nature.

The composition of fibrous material 22 is not particularly limited andmay include, for example, a polyester. In addition, the compositions ofbuffer tube 16 and fibrous material 22 may be chosen to promote adhesionthrough coupling of fibrous material 22 to a wall of buffer tube 16through the mechanical interlocking of fibers from fibrous material 22embedding into the wall of buffer tube 16 during the extrusion processof the buffer tube 16.

In particular, during the process of extruding the buffer tube, thefibers of the fibrous material 22 may become embedded in the buffer tube16 so as to mechanically couple the two. During thebuffer-tube-extrusion process, the fibrous material 22 is not bonded tothe water-swellable element 14 because adhesive material 24 has yet tobe cured. This yields the following advantages.

By using a curable adhesive material 24, which is not yet cured, andthus does not bond fibrous material 22 to water-swellable element 14during the buffer-tube-extrusion process, relative movement is permittedbetween the buffer tube and the optical fibers. Thus, the optical fiberswill not be subjected to undesirable compressive strain during coolingof the buffer tube. That is, (i) the optical fibers and water-swellableelement will sufficiently remain together (e.g., remain together insync, moving at the same rate) during the extrusion process, and (ii)the optical fibers and water-swellable element will move relative to thefibrous material and buffer tube due to the mechanical coupling (e.g.embedding) between the fibrous material and the wall of the buffer tube.

In addition, if a thermoplastic adhesive material 20 is used for bondingoptical fibers 12 to the water-swellable element 14, the use of acurable adhesive material 24, which is not yet cured, and thus does notbond fibrous material 22 to water-swellable element 14 during thebuffer-tube-extrusion process, will minimize or prevent unwantedshearing of thermoplastic adhesive material 20. In this regard, the useof a curable adhesive material 24, which is not yet cured, and thus doesnot bond fibrous material 22 to water-swellable element 14 during thebuffer-tube-extrusion process, keeps the optical fibers andwater-swellable element sufficiently together (e.g., together in sync,moving at the same rate) during the extrusion process. If it wereotherwise, and the optical fibers and water-swellable element werepermitted to move relative to one another, any adhesive between them,such as adhesive material 20, may be sheared and/or agglomerated intolumps that are undesirable in close proximity to the optical fibers.

According to the fourth aspect of the cables, curing of adhesivematerial 24 may be designed to take place downstream (e.g., after thebuffer tube has cooled to a predetermined degree). Once cured to thepredetermined degree, fibrous material 22 will be bonded to thewater-swellable element 14, and water-swellable element 14 will therebybe adhered/coupled to buffer tube 16.

According to an embodiment of a fifth aspect of the cables, illustratedin FIG. 6, a buffer tube arrangement 10 includes a plurality of opticalfibers 12, a water-swellable element 14 disposed around the opticalfibers 12, and a buffer tube 16 in which the optical fibers 12 andwater-swellable element 14 are enclosed.

An adhesive material for bonding the optical fibers 12 to thewater-swellable element 14 may be optionally included. The adhesivematerial may be any of the adhesive materials previously identified inconnection with adhesive material 20 of FIGS. 1, 3, and 5.

The fifth aspect differs from previous aspects in that thewater-swellable element 14 necessarily includes more than one layer,wherein the composition of an inner layer 26 of the water-swellableelement 14 includes a material compatible with a material forming thebuffer tube 16. In this regard, a seam 28 of inner layer 26 remainsexposed to buffer tube 16 after wrapping of the water-swellable element14. Accordingly, adhesion between the buffer tube 16 and water-swellableelement 14 may be enhanced by the compatibility of inner layer 26 andbuffer tube 16. Preferably, the material of the water-swellable elementinner layer 26 (and thus the material of seam 28) has a melt profile,(e.g., a melting point) that is substantially similar to that of thematerial forming the buffer tube 16.

As for outer layer 30 of the water-swellable element 14, it may compriseany of the materials previously identified in connection with fibrousmaterial 22 of FIG. 5. The term “outer” as used in connection with thelayers of the water-swellable element 14 relates to the layer of thewater-swellable element 14 closest to the buffer tube 16.

As an example, in one embodiment (not illustrated), a cable according tothe fifth aspect may include a water-swellable element containing twopolypropylene inner layers, having water-swellable powder sandwichedbetween the two inner layers, and a fibrous polyester resin as a third(outer) layer. The buffer tube may also be made of a material containingpolypropylene.

A seam of the polypropylene inner layer(s) remains exposed afterwrapping of the water-swellable element. Therefore, upon entering thebuffer-tube-extrusion process, the seam will be exposed to the buffertube being extruded on (i.e., around) the water-swellable element.

Accordingly, in addition to the slight amount of adhesion achievedthrough mechanical coupling between the buffer tube and water-swellableelement, which occurs due to fibrils of the water-swellable element'spolyester-containing layer (the outer layer) becoming embedded in theinner wall of the buffer tube during cooling of the buffer tube, theseam provides enhanced adhesion between the water-swellable element andthe buffer tube through bonding. In the example using a water-swellableelement having polypropylene inner layers and a polypropylene-containingbuffer tube, the seam of polypropylene from the inner layer(s) may meltinto the inside wall of the buffer tube during the buffer-tube-extrusionprocess to provide a firm bond upon cooling. Polyester has a highermelting point than polypropylene, such that the polyester-containingouter layer of the water-swellable element may not participate in thiskind of bonding.

In effect, the seam from the inner layer(s) of the water-swellableelement may function as an alternative to curable adhesive materials 18and 24 employed in the previously described aspects of the cables.However, in certain embodiments, cables according to the fifth aspectmay also employ a curable adhesive material for further bonding of thewater-swellable element to the buffer tube. The curable adhesive may beany of the curable adhesive materials previously identified inconnection with curable adhesive material 18 of FIGS. 1, 2, and 4 andcurable adhesive 24 of FIG. 5.

In the specification and figures, typical embodiments of the inventionhave been disclosed. The present invention is not limited to suchexemplary embodiments. Departures can be made from aspects andembodiments of the cables described herein by those of ordinary skill inthe art without departing from the spirit and scope of the invention.Unless otherwise noted, specific terms have been used in a generic anddescriptive sense and not for purposes of limitation.

1. A buffer tube arrangement, comprising: an optical fiber bundleincluding a plurality of optical fibers; a buffer tube enclosing saidoptical fiber bundle; a water-swellable element disposed between saidoptical fiber bundle and said buffer tube, said water-swellable elementsubstantially enclosing said optical fiber bundle; and a plurality ofdiscrete domains of adhesive material adhesively coupling saidwater-swellable element and said optical fiber bundle without requiringfrictional coupling.
 2. The buffer tube arrangement of claim 1, whereinsaid plurality of discrete domains of adhesive material provide thepredominant coupling force between said water-swellable element and saidoptical fiber bundle.
 3. The buffer tube arrangement of claim 1, whereinsaid discrete domains of adhesive material are provided on said opticalfiber bundle.
 4. The buffer tube arrangement of claim 1, wherein saiddiscrete domains of adhesive material are provided on saidwater-swellable element.
 5. The buffer tube arrangement of claim 1,wherein said adhesive material comprises a curable adhesive material. 6.The buffer tube arrangement of claim 1, wherein said optical fiberbundle comprises a ribbon stack.
 7. The buffer tube arrangement of claim1, wherein said optical fiber bundle comprises helically wound opticalfibers.
 8. The buffer tube arrangement of claim 1, wherein saidwater-swellable element comprises water-absorbing tape and/orwater-absorbing yarn.
 9. The buffer tube arrangement of claim 8, whereinsaid water-swellable element comprises water-absorbing particles. 10.The buffer tube arrangement of claim 1, wherein said water-swellableelement comprises a water-swellable powder sandwiched between twolayers.
 11. The buffer tube arrangement of claim 1, wherein the buffertube arrangement is essentially free of thixotropic filling gel.
 12. Abuffer tube arrangement, comprising: an optical fiber bundle including aplurality of optical fibers; a buffer tube enclosing said optical fiberbundle; a water-swellable element disposed between said optical fiberbundle and said buffer tube, said water-swellable element substantiallyenclosing said optical fiber bundle; and a plurality of discrete domainsof adhesive foam adhesively coupling said water-swellable element andsaid optical fiber bundle, wherein said plurality of discrete domains ofadhesive foam predominantly comprise dots of adhesive foam (i) having anaverage diameter of less than about one millimeter and (ii) being spacedfrom one another by at least about one millimeter.
 13. The buffer tubearrangement of claim 12, wherein said plurality of discrete domains ofadhesive foam provide the predominant coupling force between saidwater-swellable element and said optical fiber bundle.
 14. The buffertube arrangement of claim 12, wherein degree of foaming of said adhesivefoam is between about 1 percent and 95 percent.
 15. The buffer tubearrangement of claim 12, wherein said adhesive foam comprisesthermoplastic foam.
 16. The buffer tube arrangement of claim 12, whereinsaid adhesive foam comprises thermoset foam.
 17. The buffer tubearrangement of claim 12, wherein said adhesive foam comprises silicone.18. A method of making a gel-free buffer tube, comprising: providing aribbon stack of optical fibers; providing a water-swellable tape;applying discrete domains of adhesive foam to the ribbon stack ofoptical fibers and/or the water-swellable tape to bond the ribbon stackof optical fibers and the water-swellable tape without requiringfrictional coupling; substantially enclosing the ribbon stack of opticalfibers within the water-swellable tape, whereby the discrete domains ofadhesive foam adhesively couple the ribbon stack of optical fibers andthe water-swellable tape; and extruding a polymeric buffer tube aroundboth the ribbon stack of optical fibers and the water-swellable tape.19. A buffer tube arrangement according to claim 12 wherein saidplurality of discrete domains of adhesive foam adhesively couples saidwater-swellable element and said optical fiber bundle without requiringfrictional coupling.
 20. A buffer tube arrangement according to claim 12wherein said plurality of discrete domains of adhesive foam resistsmovement of the optical fibers relative to the water-swellable elementwithout requiring the application of an external compressive force uponthe water-swellable element.
 21. A buffer tube arrangement according toclaim 1, wherein said discrete domains of adhesive material comprisediscrete domains of adhesive foam.
 22. A buffer tube arrangement,comprising: an optical fiber bundle including a plurality of opticalfibers; a buffer tube enclosing said optical fiber bundle; awater-swellable element disposed between said optical fiber bundle andsaid buffer tube, said water-swellable element substantially enclosingsaid optical fiber bundle; and a plurality of discrete domains ofadhesive material adhesively coupling said water-swellable element andsaid optical fiber bundle, wherein said plurality of discrete domains ofadhesive material resists movement of the optical fibers relative to thewater-swellable element without requiring the application of an externalcompressive force upon the water-swellable element.
 23. The buffer tubearrangement of claim 22, wherein said plurality of discrete domains ofadhesive material provide the predominant coupling force between saidwater-swellable element and said optical fiber bundle.
 24. A buffer tubearrangement according to claim 22, wherein said discrete domains ofadhesive material comprise discrete domains of adhesive foam.
 25. Thebuffer tube arrangement of claim 22, wherein said water-swellableelement comprises water-absorbing particles.
 26. The buffer tubearrangement of claim 22, wherein said water-swellable element comprisesa water-swellable powder sandwiched between two layers.
 27. The buffertube arrangement of claim 22, wherein the buffer tube arrangement isessentially free of thixotropic filling gel.